The capture and sequestration of carbon dioxide (CO2) emissions needs to be significantly improved if the climate change consequences of such emissions are to be controlled or curtailed. The CO2 produced from combustion and industrial processes, including power plant flue gas, is perhaps the largest single greenhouse gas emission. Most existing carbon capture and sequestration methods take a two-step approach. First, they seek to separate CO2 from the flue gas or other gaseous emission source. These may include capture of the CO2 in liquid solvents, solid zeolite or various membranes. However, the capture media need to be regenerated without releasing the CO2 into the atmosphere, and this is difficult to achieve in standard physical separation processes.
The second step is sequestering the CO2 gas or liquid by inserting it into underground geological formations or in deep ocean layers. However, very specific geological configurations are required for disposal of the CO2, and these are not commonly available at CO2 emission sites. Thus, transportation adds substantial cost and difficulty. In addition, it is not known whether CO2 can be permanently sequestered underground. The two-step approach also is not economical because often CO2 represents only a small percentage of a large volume of flue gas, and treating a large flow stream to recover a small portion of it as CO2 is wasteful and expensive.
Another approach to CO2 capture and sequestration involves mining, crushing and transporting rocks to the emission site, where the crushed rock is used to absorb CO2. But this requires a good deal of heat and pressure. The energy input and environmental costs of mining the rock and transporting it to and from the CO2 source, as well as the energy costs of having the crushed rock accept and absorb the CO2, are very high.
Other ways to capture CO2 include chemical absorption using liquids such as amines or aqueous solutions of bases, physical absorption in an appropriate solution, and membrane separation. All of these methods have the problem that the absorption media need to be regenerated without losing CO2. Other capture methods such as physical adsorption and cryogenic separation require significant amounts of energy in the form of heat or pressure. Some CO2 capture methods react CO2 (or carbonic acid formed from water and CO2) with an aqueous solution of an alkali to form a carbonate. However, a significant drawback of that approach is that the carbonate exits the process in solution with water, requiring further, energy intensive treatment to separate the solids and the water, or it results in a large-volume, heavy, wet, cement-like paste that requires energy intensive drying and mechanical systems to control the size, configuration and weight of the resulting dried product. Although some are examining techniques for capturing and sequestering CO2 from ambient air, they are not suitable for CO2 emissions from power plants because of the substantial difference in CO2 concentration between ambient air and flue gas. Ambient air generally contains between about 0.03% and 0.04% CO2, whereas flue gas contains 3.0% or higher concentrations of CO2. Removing very small quantities of CO2 from the very large quantities of ambient air is not as viable and productive as the capture and sequestration of large amounts of CO2 from streams, such as flue gas, where the CO2 is more concentrated. Once the CO2 is released into the atmosphere, control of CO2 is lost. The only effective check point is at the source of CO2 generation.
Many of the same industrial processes that cause CO2 emissions also pollute the environment. For instance, heavy metals become concentrated or enriched in many industrial wastes, such as the Red Mud that is the byproduct of aluminum refining; or fly ash and bottom ash that are the byproducts of coal combustion; or ash from Municipal Solid Waste Incinerators (MSWI), where the ash is the byproduct of burned municipal waste. In all those and other similar waste streams, trace metals are present at the parts-per-million (ppm) level in small absolute concentrations. An environmental burden can be created when these metals leach from ash or Red Mud containment areas. Most of the metals found in ashes (and in Red Mud) are toxic, even at low ppm concentration levels. Chemically, such metals are members of all but two groups of the periodic table, and common examples are arsenic, mercury, lead, uranium, vanadium and nickel. This creates special needs for the disposal of fly ashes (and bottom ash and Red Mud) and establishes a significant environmental burden, beyond the liability that relates to the pH levels observed in ashes.
On the other hand, it is not unusual to find elements enriched in coal and MSWI ashes (and in Red Mud), which have significant economic value, even if they are found in small quantities. Such elements include but are not limited to the following, listed in alphabetical order: Cerium (Ce), Dysprosium (Dy), Europium (Eu), Gallium (Ga), Germanium (Ge), Lanthanum (La), Neodymium (Nd), Niobium (Nb), Terbium (Tb), Uranium (U), Yttrium (Y), and Zirconium (Zr). It is the economic value of some of these metals which makes recovery viable even at levels below 20 ppm in some cases. This includes elements such as uranium and several “rare earth elements.” For example, recent commodity prices for Germanium were listed on web-based commodity pricing sites at approximately $545/lb. Terbium was listed at approximately $364/lb. However, several lower-value elements will yield a higher revenue stream when recovered from ash, because those lower-value elements are found at higher concentrations in the ash. For example, Zirconium, Yttrium and Cerium are found in many ash streams at higher concentrations (up to 500 ppm) than, say, Europium, which can be found at 2-5 ppm. When the various recoverable elements are compared to their commodity pricing and their proportion in fly ash, Gallium, Yttrium, Zirconium, Cerium and Lanthanum are five of the most valuable recoverable elements. Rare earth elements are used in computers, photovoltaic cells, wind turbines and other renewable energy systems, hybrid cars, advanced weapons systems, and ubiquitous communications devices, among many other applications. Those uses span the full spectrum of cutting edge technologies aimed at reducing emissions and generally improving the environmental profile (carbon footprint) of many products.
Currently there are few U.S. sources of such elements. China presently accounts for well over 90% of the world's production of rare earth elements. Recent Chinese export restrictions on rare earth elements are affecting production of technology goods in Germany and Japan, demonstrating geopolitical limitations regarding raw material availability. There have been several recent proposals to re-open closed mines in the U.S., where such rare earth elements can be found in concentrations high enough to justify the mining and refining operations. The present invention offers a more efficient and less environmentally damaging way to “mine” existing waste streams, solving the following problems at once —CO2 emissions, waste stream mitigation, and rare earth element “mining.” To emphasize, the metals addressed by the disclosed processes are, irrespective of specific examples given, the metals and metalloids of all groups of the periodic table, with many of them demonstrating toxic properties or having commercial value, or both.
Waste disposal sites, also known as landfills, naturally produce landfill gas (LFG). The most common waste source accepted at landfills is household waste (“garbage”), collected by public and private trash hauling entities that serve municipalities. Some landfills also accept industrial waste, which may include construction and demolition waste (such as demolished drywall that contains sulfur compounds), as well as alkaline ashes. The LFG produced by the breakdown of the buried waste consists mostly of carbon dioxide, methane and moisture. The CO2 content of typical LFG can be above 50%. Most LFG sites either burn the methane-carrying LFG in engines (or turbines), which drive generators that produce electricity, or they flare the LFG. Either way, the CO2 content of the LFG and the CO2 that is produced by the combustion of methane is released into the atmosphere. Along with the CO2, sulfur compounds are also released, where construction waste is accepted as part of the landfill's waste stream.
Therefore, there exists a need for a commercially viable carbon capture and sequestration process that works at industrial scales, and for such sequestration to be complete and permanent. Specifically, there is a need for a carbon capture system that does not use capture media that require complex and energy-intensive regeneration, and does not yield a heavy, wet end-product that requires energy-intensive drying and other post-capture processing. There is a further need for a carbon capture and sequestration process that permanently sequesters CO2 at the site of CO2 emission. In summary, a need exists for: (1) a carbon capture and sequestration system that is cost-effective and not energy intensive and results in permanent sequestration of CO2, and (2) an energy-efficient process for converting fly ash, Red Mud and other industrial waste streams into environmentally benign materials while isolating valuable trace metals.